1. Field of the Invention
The present invention relates to a disc brake apparatus, and more particularly to a disc brake apparatus configured such that, by means of an actuator which operates in accordance with an operation force (e.g., brake-pedal stepping-on force), a braking piston which is fitted into a cylinder portion of a caliper in an axially slidable manner is axially driven so as to push a pad toward a disc rotor; a friction force in a circumferential direction of the disc rotor between the pad and the disc rotor is taken out as an anchor load (braking reaction-force); and a servomechanism converts the anchor load to a servo load, which is applied to the braking piston.
2. Description of the Related Art
A disc brake apparatus of this kind is disclosed in, for example, Japanese Patent No. 3000835. In this disc brake apparatus, an anchor load is taken out in the form of oil pressure, and the oil pressure is applied to a braking piston via a pressure-increasing control valve of a servomechanism.
The pressure-increasing control valve appearing in the above publication functions as a normally-open on-off valve. When the taken-out oil pressure becomes a predetermined number of times an operation force (i.e., master cylinder oil-pressure) or more, the pressure-increasing control valve shuts off an oil-pressure transmission path to the braking piston, thereby preventing application of the taken-out oil pressure to the braking piston. This prevents an increase of an oil pressure (servo oil-pressure) to be applied to the braking piston to a predetermined value or higher.
However, since the oil pressure (servo oil pressure) which is applied to the braking piston through the pressure-increasing control valve is merely an oil pressure converted from the anchor load, the following problem arises. When coefficient of friction between the pad and the disc rotor varies because of, for example, entry of water or thermal fading, the oil pressure varies with the coefficient of friction. Specifically, when coefficient of friction between the pad and the disc rotor decreases (increases) as compared with a predetermined value, the friction force between the pad and the disc rotor decreases (increases) as compared with a predetermined value, and thus the anchor load decreases (increases) as compared with a predetermined value. Accordingly, the servo oil-pressure decreases (increases) as compared with a predetermined value, and thus the servo load to be applied to the braking piston decreases (increases) as compared with a predetermined value. Thus, the braking force which the disc brake apparatus produces decreases (increases) as compared with a predetermined value. As a result, the disc brake apparatus fails to brake the disc rotor in a predetermined braking mode (a braking mode for the case where the coefficient of friction is the predetermined value). In the case of a vehicle, the vehicle fails to be braked within a predetermined braking distance.